Technically speaking, in order for this to work properly, the chosen song must fit the following criteria:
1. Little or No changing of the original lyrics. (More people can sing along instantly, and it removes the cheese factor)
2. The tune must be carried in the vocals and not rely on instruments or percussion.
3. Song should be well known.
4. Slow paced songs work best.
I really like stand by me, it has a great sentiment, but it'd be too difficult for 40,000 people to sing in unison - and it would almost certainly rise in tempo with no backing track. When you hear You'll Never Walk Alone, they sing it slowly and passionately.
The best short term solution would be to sing Ghost Riders (Holte Enders) Chorus over and over at the tempo of the original - I think that would be really haunting.